1. Field of the Invention:
This invention relates to a control apparatus for maintaining braking force in an automotive vehicle controlled by an electronic control unit, particularly an automotive vehicle equipped with an automatic transmission with a friction clutch controlled by an electronic control unit.
2. Description of the Related Art:
When a vehicle is to be stopped on an upgrade and then moved forward from rest, the driver first engages the hand brake with one hand to stop the vehicle. Then, while engaging the clutch, the driver steps down on the accelerator pedal and slowly releases the hand brake to allow the vehicle to move forward. In a case where the vehicle cannot be stopped on the upgrade effectively because the braking force applied by the hand brake alone is insufficient, the driver steps down on the brake pedal with his right foot to stop the vehicle. The driver then withdraws his left foot from the clutch pedal and, while this is being done, transfers his right foot from the brake pedal to the accelerator pedal to raise the engine rpm. As the engine rpm is being raised, the vehicle is propelled forward when the clutch is engaged in concurrence with removal of the driver's foot from the brake pedal. The changeover between the brake pedal and accelerator pedal and the operation to engage the clutch must be performed at a certain timing or else the engine will stall. These operations require that the driver have a high level of skill.
In order to eliminate this inconvenience, the applicant has proposed a control apparatus for maintaining braking force disclosed in the specification of Japanese Patent Application Laid-Open No. 60-12360 cited hereinabove as a cross-reference.
This previously disclosed control apparatus is applied to an automotive vehicle having an automatic transmission equipped with a friction clutch, that is controlled by an electronic control unit and is adapted to collectively interpret signals from various vehicle sensors, maintain braking force when the vehicle is stopped by operation of a brake, and hold the braking force until a clutch is engaged by operation of a clutch actuator when the vehicle is propelled forward from rest. This makes it possible to start the vehicle moving forward again on an upgrade without requiring that the driver have a high level of skill.
When a braking force is applied at such time that the vehicle is traveling on a road surface having little friction, as when the road surface is covered with snow or ice, the wheels may lock and allow the vehicle to continue skidding along the road even though the wheels have stopped. An extremely hazardous situation can result if the control apparatus does not distinguish between this state and the state in which the vehicle is truly at rest. In order to prevent erroneous operation in terms of the braking force maintaining function, the conventional apparatus is designed to recognize the wheel locking state based on the rate at which vehicle velocity decreases.
More specifically, when the rate of decrease in vehicle velocity is high at braking, wheel locking is judged to be occurring and the braking force holding function (HSA) is inhibited when the driving system for the wheels is stopped. When a signal indicative of brake pedal depression vanishes, namely when the braking force is removed, the inhibition on the HSA is canceled. Consequently, when the brake pedal is stepped down on repeatedly in a short period of time, namely when the brakes are "pumped", the inhibition on the HSA is removed immediately after the braking force vanishes, even if wheel locking is detected. As a result, the HSA may operate erroneously due to a time delay in the electronic control unit.